Method and materials for control of contact electrostatic development

ABSTRACT

Magnetically attractable conductive carrier particles such as iron, are treated with resins such as, for example, nitrocellulose, acrylic polymers, polyvinyl chloride polymers and chlorinated rubber. The resin as applied constitutes 2 percent by weight of the weight of iron. The treatment changes the triboelectric charge generated when the toner particles carried on the resin coated iron are separated by attraction to an electrostatic image area. The charge generated is in the range of 0.4 volts to 4.0 volts per milligram of iron and toner mixed and its magnitude will depend on the type of resin. The resin treatment also affects the direction of the toner, i.e., either positive or negative relative to the image area to be developed.

nited States Patent 1191 i Browning 1111 3,811,80 1451 May 2 11, 1974 [75] Inventor: Luther C. Browning, Palatine, Ill.

[73] Assignee: Addressograph-Multigraph Corporation, Mount Prospect, Ill.

22 Filed: Mar. 20, 1972 21 Appl.No.:238,-699

- Related [1.8. Application Data [63'] Continuation of Ser. No. 35,884, May 8, 1970,

abandoned.

[52] U.S. C|..;..'. 96/].4, 252/611, 117/175 [51] Int. Cl G03g 13/08 [58] Field Of Search 252/621; 117/175, 100; 96/14 [56] I References Cited UNITED STATES PATENTS 2,874,063 2/ 959- orei 117/175 2,890,968 6/1959 Giaimo 117/175 3,507,686 4/1970 Hagenbach 117/100 5 3,640,751 Kasuya et al. 252/62.l

FOREIGN PATENTS OR APPLICATIONS 572,459 3/195'9 Canada ..2S2/62.1 1,174,571 12/1969 Great Britain Primary Examiner-Ronald H. Smith Assistant Examiner-John L. Goodrow [57] ABSTRACT Magnetically attractable conductive carrier particles such as iron, are treated with resins such as, for example,'nitrocellulose, acrylic polymers, polyvinyl chlorand toner mixed and its magnitude will depend on the I type of resin. The resin treatment also affects the direction of the toner, i.e., either positive or negative relative to the image area to be developed.

2 Claims, No Drawings METHOD AND MATERIALS FOR CONTROL OF CONTACT ELECTROSTATIC DEVELOPMENT This application is a continuation of Ser. No. 35,884 filed May 8, 1970, now abandoned.

' BACKGROUND OF THE INVENTION Electroscopic particles, by definition, are charge seeking. In order to;have the electroscopic particles, or toner powder as it is generally known in this art, adhere to the charged areas, use is made of triboelectric forces.

According to well-establishedprinciples, two different materials when rubbed together generate static charges. The creation of positive or negative electrical charges by mechanical action are known as triboelecthat they tend to be deposited on the paper and be carried out of the system. This means that the amount of the metal soap additive varies greatly and ultimately the developer mix becomes exhausted.

Another problem with magnetic brush systems is the control of the strength of the attractive force between the carrierand the toner which is an important factor in the density of the image that is developed. The strength of the field in the image area is the attractive force which pulls the toner from the carrier. A low potential in the image area and a high attractive force or bond between the carrier and the toner will result in a low to poor density copy.

In copying systems in which the developed powder image is transferred to a receiving sheet, the ease of transfer of the toner particle will be governed by the force with which the particle is retained on the photoconductive area. The force of retention is dependent on the extent to which the particle neutralizes the charge in the image area.

SUMMARY OF THE INVENTION iv It has been found that by treating the conductive tric charges. An elementary explanation of the triboelectric phenomenon is the exchange ortransfer of charges between two dissimilarmater-ials in intimate contact which are then separated. The materials are neutral, that is, uncharged before contact. I

It is generally known in the art that glass beads or steel beads can be treated with various resins for the purpose of preventing the attraction of the carrier particles to the image area, such as disclosed in U.S. Pat.

No. 2,6l8,55l. The treatment of carrier particles in this prior art patent is for the purpose of preventing the carrier from adhering to the charge and uncharged areas of the plate and to insure removal of the carrier from the photoconductive layer.

Untreated carriers have been used with electroscopic powders in the preparation of developer mixes. The performance of such mixes is largely controlled by varying the electroscopic powder formulations by including various additives in order to cause the powder to be attracted to either a positively charged image or a negatively charged image. These additives include such materials as organic dye stuffs which function as orienting agents or fatty acid derivatives included as separate materials for the purpose of improving the longevity of the developer material.

Magnetic brush type developing systems which employ magnetically attractable carrier particles in toner particles as the developer mixture suffer from certain deficiencies. One such deficiency is the fatigue of the system which is best defined as the fall off of the triboelectric forces which causes the system to lose either its positive or negative orientation. The existence of fa- 'tigue of a particular mix is manifest by the indiscriminate deposition of the toner particles in-both the image and the non-image areas. The problem of "fatigue has been placed under control by the use of metal soap ad-' ditives, but these materials are fugitive in thesystem in magnetically attractable carrier'particles with various resin coatings the deficiencies heretofore experienced with magnetic brush systems can be brought under control. For the purposes of this discussion, the term of magnitude will relate to the relative strength of the carrier-toner bond and the term direction will mean the polarity of the charge carried by the toner particle after it is separated from the carrier.

While the prior art teachings disclose atreatment of carrier particles, it has been found that in the method of practicing magnetic brush development, the treatment of the magnetically attractable carrier particles greatly enhances and improves the ease with which the direction of the toner particle may be controlled, the life of the developer mix and'the ease of transferring the toner particle to a receiving sheet.

It is therefore'a general object of this invention to provide an improved electrostatic developing method wherein the developer mix contains treated carrier particles for controlling the triboelectric forces giving improved image formation with the electrostatic printing process.

It is a further object of this invention to provide an improved electrostatic developing process in which the carrier particles of a developer mix are treated in order to improve the triboelectric relation between the carrier and the toner so that a wide variety of electroscopic powders may be used with a given magnetically attractable carrier.

It is a specific object of the present invention to provide van improved electroscopic printing process whereby the triboelectric forces are controlled by treating the carrier material to produce a developer mix having improved performance in an image transfer system.

It is yet a further specific object of the instant invention to provide improved developer mixes which may be either positively or negatively oriented by treating the carrier particles and thereby controlling the triboelectric forces.

It is a still further specific object of the instant invention to provide developer mixes suitable for magnetic brush systems in which the longevity of the developer mix is greatly extended.

In carrying out the object of the instant invention, the treatment is limited to carrier particles which are magnetically attractable for the reason that such materials are usually conductive and behave as electrodes in the environment of a differential charge image pattern on a photoconductive sheet. Because the carrier particles are conductive, they affect the electrostatic fields which emanate from the charged image area causing the fields to follow a straight line pattern and hence promote large area development. Because of the role it plays as an electrode in the developer mix system, treatment of the conductive carriers tends to preserve their electrical properties, such as the bulk resistance of the materials which is deemed to be a factor in stabilizing the triboelectric effects. Once having stabilized or fixed the properties of the carrier by the treatment of this invention, continued performance as carriers as well as the direction of the charge carried by the toner and the relative strength of the carrier-toner bond is stabilized and hence the performance of the developer v mix becomes more predictable and reliable.

The carrier particles have been described in the prior art to cover a wide range of sizes, such as from 20 microns to 200 microns in size. From the standpoint of this invention, the size of the carrier particle is not critical. In the preferred construction, the carrier particle should be larger than the toner particle with which it will make contact. However, this is not essential. Carrier particles may be selected from a wide variety of magnetically attractable materials including but not limited to iron powder, steel shot or filings, magnetite or ferrite particles, or other finely divided magnetic material.

The toner particles are made up of a resinous material and depending on the fixing technique whereby the image is secured or made permanent to the receiving sheet would fully determine the exact make-up of the toner.

The technology of formulating electroscopic powder is well developed. As in the following, it is a typical electroscopic toner-powder formulation that may be used in carrying out the instant invention,

Polyamide resin 32.5% by weight Versamide 930 available from General Mills Corp,

Polyhydric alcohol 9.7% by weight Shell X-450 Shell Development Corp.

Maleic anhydride modified rosin 48.9% by weight Kruhmbaar K-l8l3 Lauter Chemical Company Carbon black 2.38% by weight 4 PCO 5 black Capital Color Corporation Black Dye Stuff 6.52% by weight Nubian rcsin black Keystone Aniline and Chemical Corporation The following is a list of typical materials to be used in treating the carrier particles. This list is merely exemplary and not intended to be exhaustive of the various materials that may be employed to achieve the results of the instant invention.

TREATING MATERIAL DIRECTION Positive Negative Positive Negative Positive Cellulose Esters Nitrocellulose A, Positive Allyl Ester Polymers Poly dimethyl di-allyl ammonium chloride Negative Vinyl benzyl polymer Negative Polyvinyl benzyl trimethyl ammonium chloride Chlorinated rubber Positive Parlon from Hercules Powder Co.

Polycarbonates Positive Polymers derived from direct reaction between aromatic and aliphatic dihydroxy compounds (General Electric Company Hexon Grade I45) Polyolefin Positive CPR l, Union Carbide Company Polyurethane Positive Derived from reaction of diisocyanates with glycols Estane" sold by B. F. Goodrich No. 5702-l I. Vinyl chloride esters Positive (l) Polyvinyl chloride Polyvinyl Butyral (XYHL sold by Union Carbide Bakelite) (2) Polyvinyl chloride acetate (VMCA sold by Union Carbide Bakelite) .l. Epoxy resins Positive Epoxy plus a polyamide activator (Versamide I40) Olefinic resins Positive Copolymer of styrene and butadiene manufactured by Goodyear Tire and Rubber Company under the tradename PLIOLITE S-SA.

In preparing the carrier particles of the instant invention, materials may be applied to the iron by several different techniques: (1) precipitation from solution by a non-solvent such as petroleum ether; (2) spraying of a solvent solution onto the iron particles and evaporating the solvent; (3) immersion of a mass of carrier particles in a solvent resin solution, agitating the slurry and then evaporating the solvent from the mixture.

To achieve the results of the instant invention, it is only necessary that the polymer be thinly applied so that it forms a very thin coating over each of the particles. The thickness of the application or its uniformity of the coating is not critical and it is only necessary that a sufficient amount be applied to coat substantial portions of each of the individual particles. Too thick a coating will cause agglomeration of the iron particles which is undesirable since it is necessary for the sucamples and that weight'basis. I

cessful operation of magnetic brush systems that the particles that comprise the granular mass remain freeflowing in their coated condition. In general the amount of resin coated on the. carrier particles should be in the range of about 0.5 to 2.0 percent by weight based on the weight of magnetically attractable carrier particles.

In order to determine the effect of the treatment on modifying the direction of the carrier particle in relation to the electroscopic powder, measurement is made of the triboelectric charge generated when the electroscopic powder is separated from the carrier. The measurement is made by determining the voltage generated by impinging the developer mix onto a wire screen such that the toner escapes through the screen but the carrier is retained.

The polarity of the observed'voltage is related to the polarity of the charge which is imparted to the toner, while the magnitude of the voltage per unit mass of toner separated indicates the amount of charge acquired by the separated toner. A large voltage per mass is indicative of a strongcarrier-toner bond. As a result of experimentation in magnetic brush formulations such a strong bond is evidenced by values in the range of 1.5 to 5 volts per milligram.

In the range of 0.5 to 1.5 volts per milligram the carrier to toner bond may be characterized as being weak.

The following examples are given to illustrate preferred embodiments and processes for producing developer mixes embodying this invention. It will be understood that this invention is not limited to these ex- I they are merely illustrative of the invention.

In these examples, all percentages are given on a EXAMPLE l The developer mix of this Example was prepared using the toner formulation described above.

The carrier is prepared by using powdered iron such as sold by the Hoganese Metal Company, Grade MI-I-l. The powdered iron was treated by mixing 600 grams of the material with 12 grams of nitrocellulose dissolved in I00 cc. of acetone.

The iron and resin solution were combined and mixed together in. a jar mill until the solution was uniformly distributed throughout the iron powden' After tumblingt'he mixture for 30minutes, the contents were spilled into a shallow pan and the solvent evaporated by the use of a heated forced air.

The treated carrier particles, which now were coated with nitrocellulose, were combined with the toner formulation described above in the ratio of 37 parts by weight of carrier per 1 part by weight of toner. The mixture of toner-and carrier were also tumbled together in a jar mill for the purpose of uniformly mixing together the two ingredients. The mixture was charged into an electrostatic copier which utilizes a magnetic brush. A positive original was used and a zinc oxide electrophotographic member was employed, to which was imparted a negative electrostatic charge. The treatment to the carrier was positive directing which resulted in thedevelopment of a positive image from a positive original. I

In order to determine the direction and magnitude of the treated carrier, a weighed amount of the developer mix is charged into one end of a metal cylinder adapted to receive a flow of air or inert gas such asnitrogen, the other end being covered by a 325 mesh screen. The sample cylinder is concentrically positioned inside another conductive cylinder which is attached to an electrometer. The test cylinder is exposed to a passage of agiven volume of nitrogen for a controlled period of time forcing the toner particles to pass through the wire screen while retaining the larger carrier particles. The toner particles in the act of being separated from the carrier particles generate a voltage which is induced in the adjacently located metal screen and metal cylinder which is then recorded on the volt meter. The induced generated voltage is recorded in terms of volts per milligram of developer mix inside these test cylinders.

The electrometer also records the polarity of the charge on the electroscopic powder. In the instant Example, the electrometer recorded a positive voltage of 3.0 volts per milligram of developer mix.

EXAMPLE II A developer mix was prepared using the toner formulation set forth in Example I. An iron carrier such as used in Example I was treated with a solution comprising a cationic polymer of polyvinyl benzyl trimethyl ammonium chloridesold by Dow Chemical Corpora- EXAMPLE III A developer mix was prepared following the formulation of Example II with the exception that the treatment for the iron consisted of a 5 percent by weight solution of a terpolymer of styrene, ethyl acrylate and glacial acrylic acid polymerized using conventional polymerization techniques.

, The developer mix which resulted from the treatment of iron with the terpolymer was positive orienting and produced a measured electrostatic bond of 1.3 volts per milligram.

EXAMPLE IV 600 grams of iron carrier powder such as was used in the preparation'of the developer mix in Example I was treated with a terpolymer of ethyl acrylate, vinyl toluene and diethyl amino ethyl methacrylate, polymerized using conventional polymerization techniques, in place 'of nitrocellulose. 12 grams of the terpolymer was dissolved in cc. of methyl ethyl ketone and mixed with the carrier in a jar mill as described in the previous Examples. This treatment represented a 2 percent by weight of resin based on the total weight of iron carrier.

After the terpolymer was coated with the resin and the excess solvent evaporated, it was combined with toner in a ratio of 37 parts of iron to one part toner.

The direction of the toner in this system was negative and the force of the electrostatic bond between toner and carrier was 0.9 volts per milligram.

EXAMPLE V In this Example a quantity of iron powder, such as was used in Example I was treated with a terpolymer of ethyl acrylate, styrene and 2-vinyl pyridine, polymerized using conventional polymerization techniques, substituted for nitrocellulose. 9 grams of the terpoly mer were dissolved in I cc. of methyl ethyl ketone and mixed with the carrier particles in a jar mill following the procedures of the previous Examples. The terpolymer treatment represents a 1.5 percent by weight of resin based on the total weight of iron carrier. The mixture was forced air dried to remove excess solvent and combined with toner in a ratio of 37 parts by weight of iron to one part toner.

The system was positive directing and the magnitude of the electrostatic bond was 0.87 volts per milligram.

EXAMPLE VI tional techniques.

The system was positive directing and the magnitude of the electrostatic bond was 0.87 volts per milligram.

EXAMPLE VII EXAMPLE IX A positive directing system was prepared in this Example following the procedures set forth in Example I with the exception that the iron powder was treated with the terpolymer of ethyl acrylate, styrene and N- vinyl carbazole in place of the nitrocellulose, and the resin comprised 2 percent of the weight of iron powder.

The electrostatic attractive force between the iron and toner was 1.0 volts per milligram.

7 EXAMPLE X A negative directing system was prepared in this Example following the procedure of Example I with the 4 exception that poly methyl diallyl ammonium chloride resin was substituted for the nitrocellulose. The

strength of the electrostatic bond measured 1.5 volts per milligram.

EXAMPLE XI I and sold under the tradename PARLON. The electro- The developer mix prepared in this example followed the procedure of Example I with the exception that the iron was treated with styrene-butadiene copolymer, a

resin manufactured by Goodyear Corporation and sold under its tradename PLIOLITE S-5A.

The developer mix was strongly positive orienting and the measured electrostatic bond was 2.5 volts per milligram.

The developer mix prepared according to this Example was placed in a magnetic brush system and used to develop zinc oxide-type electrophotographic members. Approximately 3 pounds of the developer mix was deposited in the magnetic brush device. Reproductions were made using the treated iron of this Example in an electrostatic copier of a type that is commercially available. The developer mix produced in excess of 20,000 acceptable copies without the mix showing signs of a change in its performance compared to its performance during the start of the copy making run.

EXAMPLE VIII In this Example a negative directing toner was prepared by treating the iron carrier particles of the previous Examples with a copolymer of ethyl acrylatestyrene. To 600 grams of iron was added a solution of 12 grams of the copolymer in I00 cc. of methyl ethyl ketone.

The iron was coated uniformly following the procedure set forth in Example I.

The direction of the system was negative and the electrostatic attractive force was 1.1 volts per milligram.

static bond between the treated carrier and the toner was 1.6 volts per milligram of developer mix.

EXAMPLE XII In this Example a positive directing system was formulated following the procedures set forth in Example I with the exception that the resin employed was a copolymer of vinyl chloride and vinyl butyral which is a product sold by Union Carbide Bakelite under the tradename identification XYI-IL.

The strength of the electrostatic bond measured 1.3 volts per milligram of developer mix.

EXAMPLE XIII The strength of the electrostatic bond measured volts per milligram of developer mix.

EXAMPLE XIV A positive directing system was prepared in this Example by treating the carrier particles of Example I with an epoxy resin such as the resin sold by General Mills under the tradename GENEPOXY 190. In preparing the treating solution, 8.4 grams of the epoxy was dissolved in I00 cc. of methyl ethyl ketone. .To the solution was added 3.6 grams of a curing agent which was a polyamide resin manufactured by General Mills.

After mixing the iron powder with the treating solution, the coated iron was heated at C. for a period of 20 to 40 minutes in order to complete the curing reaction.

A new dimension of control is imparted to electrostatic printing processes embodying the instant invention. It will be appreciated that in the heretofore known systems, there were inherent limitations with respect to the quality of the image that could be obtained within a given triboelectric system. Accordingly, in the use of untreated iron with a specific toner formulation, there existed an unalterable electrostatic bond between the carrier and the toner particle. In the circumstance that the toner formulation was such that it gave extremely good quality images but the electrostatic bond was such that-it was unusually strong, such as for example, in excess of 1.5 volts per milligram of developer mix, the electrostatic images would nonetheless be weak and unacceptable.

In utilizing the instant invention in such a circumstance, the carrier particle could betreated with a resin such as disclosed in Example IV, V or VI and thereby modify the electrostatic bond between the toner and the carrier so that it is more easily responsive to a given charge level in the image area.

Another condition which the instant invention may i be utilized to correct is the circumstance in which the change pattern on an electrophotographic member exhibits a low differential between the image and nonimage areas. As for example in organic photoconductive systems, it is often desirable to partially discharge the background area so that there remains substantial voltage levels in the background area. These voltage levels'may range from 100 to 300 volts and would take on the character of 'an image area relative to the low electrostatic bond between a toner and carrier particle.

' Utilizing the instant invention, the electrostatic bond may be increased by treating the iron particles in accordance with Example I which would produce an electrostatic bond of 3 volts per milligram, or as high as 4.0 volts per milligram by using polyethylene resin, thereby vitiating the residual charge-in the background area. This would result in the development of the corresponding image areas which are at a level of 800 to 900 volts, great enough to attract the toner from the carrier particle. Relative to an electrostatic bond of 3 or 4 volts per milligram, the background area would have an insufficient attractive force so as to develop an image with high contrast. It will be recognizedthat while in the instant invention the Examples and the general environment have been described with respect to a particular toner formulation, other toner formulations may be used and by treating the iron, a wide range of effects can be achieved to effectively develop image areas which have a low voltage level or a high voltage level or electrophotographic or electrostatic images in which there is a low differential between the image and non-image areas. Further, electroscopic toner formulations which heretofore were found unusable because of the restrictions inherent in using untreated iron and depending solely on the triboelectric relationship between the iron and the specific toner may now come into prominent usage by virtue of the new parameter introduced by the instant invention.

In transfer systems where the powder image is trans-.

ferred to a receiving sheet it is now possible to develop images of high density on the electrophotographic member and then transfer substantially all of the powder image by minimizing the retentive forceon the parrier is treated with a resin that imparts an electrostatic attractive force in the range of 0.4 volts to about 1.0 volts per milligram of developer mix. At these low attractive force levels the toner is readily separated from the carrier and also serves to neutralize the sensitizing charge so that the retentive force is minimized greatly enhancing the effect of the potential gradient imposed between the receiving sheet and the conductive backing of the electrophotographic member. In this manner, more complete image transfers are achieved. What is claimed is: a 1. In a process for developing electrostatic images on an electrophotographic member comprising the steps of:

charging an electrophotographic member, exposing the charged electrophotographic member to a pattern of light and shadow, producing differentially charged portions comprising image and non-image areas thereon and forming a developed image on said electrophotographic member by' attracting electroscopic toner particles from carrier particles to which they are attracted by triboelectric forces with a force produced from said differentially charged portions sufficient to overcome said triboelectric forces between said toner particles and said carrier particles, causing said toner particles to be attracted to said image areas, the charge in the non-image areas being insufficient to overcome said triboelectric forces, the improvement which comprises using carrier particles comprising a magnetically attractable conductive core and a coating consisting essentially of about from 0.5 to

2.0 percent by weight of said core of a resin selected from the group consisting of acrylic resins,

cellulose esters, polyamines, chlorinated rubbers, polycarbonates, polyolefins, polyurethanes and epoxy resins,

said toner particles being attracted to said carrier particles by triboelectric forces in the range of about from 0.4 volt to 4.0 volts per milligram of the mixture of carrier particles and toner partirier and toner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTWN Patent No. 3,811,830 Dated May--21, 1974 Inventofl!) ther Browning It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 8, line 16, should read as follows:

exception that polydimethyl diallyl ammonium chloride Column 9, line 18, should read as follows:

charge pattern on an electrophotographic member ex- 7 Signed and sealed this 26th day of November 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. 0. MARSHALL 1mm Attesting Officer Commissioner of Patents FORM P0-1050 (10-69) USCOMM-DC eoanmes r US GOVERNMENT PRINTING OFFICE 1969 0-366-334 

2. A carrier for an electroscopic toner comprising a magnetically attractable conductive core and a coating consisting essentially of about from 0.5 to 2.0 percent by weight of said core of a resin selected from the group consisting of acrylic resins, cellulose estErs, polyamines, chlorinated rubbers, polycarbonates, polyolefins, polyurethanes and epoxy resins, the triboelectric attractive forces between said carrier and said toner being in the range of about 0.4 volt to 4.0 volts per milligram of the mixture of carrier and toner. 